Propulsion drive system for a utility vehicle

The invention concerns a propulsion drive system for a utility vehicle having an internal combustion engine, a hydraulic pump in drive connection with the internal combustion engine, whose fluid displacement can be varied by a first actuator, a hydraulic motor connected to the hydraulic pump by a line conducting hydraulic fluid where the fluid displacement of the hydraulic motor can be varied by a second actuator and being connected so as to drive at least one propulsion device in engagement with the ground. A control arrangement is connected with the first actuator, the second actuator and a speed input arrangement arranged to generate a target speed signal which is connected and can be operated in such a way as to control the first actuator and the second actuator so that the propulsion device is driven at the target propulsion speed as provided as input by the speed input arrangement.

In many utility vehicles, such as agricultural vehicles and more particularly harvesting machines, hydraulic drives are being utilized. These drives include a hydraulic pump driven by an internal combustion engine and a hydraulic motor connected to the hydraulic pump by lines conducting hydraulic fluid so that the hydraulic motor drives one wheel or more wheels. In some vehicles the wheels of the front and rear axle are driven hydraulically by at least one hydraulic motor associated with the axles or the wheels.

A propulsion drive system of this kind is described by DE 102 11 799 A. Here the swash plate of the adjustable hydraulic pump is controlled by an actuator and a control arrangement controls it as a function of the position of the propulsion control lever adjusted by the operator, so that the hydraulic flow demanded by the increasing speed required by the operator is generated. Two adjustable hydraulic motors are associated in each case with the front or rear axle and also include adjustable swash plates controlled by actuators connected with the control arrangement. The position of the swash plates of the hydraulic motors is controlled on the basis of the output rotational speed of the hydraulic motors and the pressure difference between their inlet and outlet, so that in the case of spinning wheels or wheels rotating opposite to the desired direction the control arrangement can correct the rotation.

In the case of the drive system according to DE 102 11 799 A the disadvantage is seen in the fact that the efficiency of the hydraulic drive consisting of the hydraulic pump and the hydraulic motor is relatively poor, particularly in the lower speed range.

DE 103 50 308 A describes a process for the selection of the engine rotational speed and the gear ratio for a continuous hydrostatic torque division gearbox that includes a hydrostatic gearbox with a constant volume unit and an adjustable volume unit as well as a summing gearbox and an area gearbox with several selectable gear ratios, there the torque provided as input to the gearbox is divided between the hydrostatic gearbox and the area gearbox and is then combined in the summing gearbox. The engine rotational speed and the gear ratio of the torque division gearbox are controlled automatically in such a way that an optimum overall efficiency results in a desired speed. The gearbox described here includes only one adjustable hydraulic element (pump or motor), so that the engine rotational speed control must be utilized in order to fulfill the two conditions (attainment of the desired speed and attainment of the highest possible efficiency). Accordingly, this arrangement is not appropriate for self-propelled harvesting machines whose internal combustion engine is operated at a fixed predetermined rotational speed at least during the harvesting operation. Moreover, in contrast to the gearbox according to the class the torque division gearbox used here provides an additional mechanical branch.

The purpose underlying the invention is seen in the need to make available an improved propulsion drive system for a utility vehicle that does not include the aforementioned problems.

The propulsion drive system of the utility vehicle, which in particular may be a self-propelled harvesting machine, includes an internal combustion engine. The internal combustion engine drives a hydraulic pump directly or indirectly, that is, via an intervening mechanical, hydraulic or any other desired gearbox. The hydraulic pump is connected with a hydraulic motor via a hydraulic line, the hydraulic motor mechanically drives a propulsion device in engagement with the ground, in particular a wheel or a crawler drive. The fluid displacement of the hydraulic pump and the hydraulic motor can be varied by a first actuator or a second actuator. A control arrangement is connected with both actuators and a speed input arrangement that provides a target speed signal which contains information regarding the desired target speed with which the utility vehicle is to be driven. The target speed input arrangement can be actuated by the operator or it operates automatically in order, for example, to attain a constant harvested crop throughput. The control arrangement has two degrees of freedom in order to attain the target speed, that is, the positions of the actuators with which the fluid displacement of the hydraulic pump and of the hydraulic motor can be varied. One of the degrees of freedom is established by the desired target speed, while the present invention proposes that the other degree of freedom be utilized in such a way that an optimum overall efficiency (that is, a maximum efficiency) of the hydraulic system consisting of the hydraulic pump and the hydraulic motor is attained. In this way the result is an improved efficiency of the drive system and a reduction of the fuel consumption.

In a further development the invention proposes that, in addition, the control arrangement be connected with an engine control arrangement of the internal combustion engine. The control arrangement also varies the rotational speed of the internal combustion engine in such a way that the overall efficiency of the entire propulsion drive system including the internal combustion engine is optimized.

Further input parameters that could be included here are the torque developed by the hydraulic motor as determined by means of a torque sensor that could be used to determine the rotational speed of the internal combustion engine required to deliver the necessary torque or power.

This variation of the rotational speed of the internal combustion engine is particularly useful during operation on public roads where a switch for the changeover between operation on a field and operation on public roads can be seen as particularly useful. On the other hand during operation on a field it is useful, as a rule, to control the rotational speed of the internal combustion engine by an operator and to hold it constant (or with an automatic control based on appropriate sensor values, for example, for the immediate harvested crop throughput) in order to make available appropriate rotational speeds for the operating devices of the operating machines, such as threshing and separating arrangements for combines or chopper and conveyor arrangements for forage harvesters.

The efficiency of hydraulic pumps and hydraulic motors is a function, in particular, of the pressure difference between the inlet and the outlet which, in turn, is a function of the fluid displacement that depends upon the position of the swash plate as controlled by the actuator. In general the efficiency of the hydraulic components increases with increasing pressure up to a specific value for the hydraulic pump or the hydraulic motor. Accordingly, a solution is to command the hydraulic pump and the hydraulic motor in such a way that they both operate at the highest efficiency and that the control arrangement be connected with a pressure sensor that is arranged to measure the pressure at the outlet side of the hydraulic pump, and adjusts the actuators in such a way that a pressure is attained at the pressure sensor at which the overall efficiency of the hydraulic pump and the hydraulic motor is at a maximum. This pressure can be stored in a memory arrangement connected with the control arrangement. Thus the fluid displacement is reduced starting from a maximum fluid displacement until the optimum pressure has been reached. Analogously the fluid L ( displacement is increased starting from a fluid displacement that is too low which brings about a pressure that is higher than the optimum pressure until the optimum pressure has been reached. This optimum pressure may be a function of the desired rotational speed of the hydraulic motor, so that the rotational speed of the hydraulic motor or the target speed can be utilized for the selection of the pressure. For this purpose appropriate curves, formulae or tables could be stored in a memory in the control arrangement and subsequently recalled.

Moreover the invention proposes that the torque developed by the hydraulic motor be detected by a torque sensor. This would permit situations to be recognized in which the pressure made available by the hydraulic pump is no longer sufficient to make the required torque available at the outlet of the hydraulic motor. In such cases in which the torque detected exceeds a threshold value, the control arrangement will induce the first and the second actuators to increase the fluid displacement of the hydraulic pump and the hydraulic motor in order to be able to make the required torque available (although at the cost of a lower efficiency). The pivot angle of the swash plate of the hydraulic pump can be repositioned up to the maximum possible value or to a lower value, but that may be sufficient to supply the required torque and that offers a better efficiency than the maximum pivot angle of the swash plate.